Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, which includes a magnetic cell core supported by a substrate. The magnetic cell core includes at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region in-between. The fixed region includes a magnetic material that has a fixed (e.g., a non-switchable) magnetic orientation, while the free region includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration, in which the magnetic orientation of the fixed region and the magnetic orientation of the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively), and an “anti-parallel” configuration, in which the magnetic orientation of the fixed region and the magnetic orientation of the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively).
In the parallel configuration the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements, i.e., the fixed region and free region. This state of relatively low electrical resistance may be defined as a “0” state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements, i.e., the regions of magnetic material, e.g., the fixed region and free region. This state of relatively high electrical resistance may be defined as a “1” state of the MRAM cell. Switching of the magnetic orientation of the free region and the resulting high or low resistance states across the magnetoresistive elements enables the write and read operations of the conventional MRAM cell. Ideally, the amount of programming current required to switch the free region from the parallel configuration to the anti-parallel configuration is essentially the same amount of programming current required to switch from the anti-parallel configuration to the parallel configuration. Such equal programming current for switching is referred to herein as “symmetric switching.”
The free regions and fixed regions of STT-MRAM cells may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) with the width of the regions. In STT-MRAM cells that have perpendicularly-oriented magnetic regions, the magnetic materials exhibiting the vertical magnetic orientation may be characterized by a strength of the magnetic materials' perpendicular magnetic anisotropy (“PMA”). The strength (also referred to herein as the “magnetic strength” or the “PMA strength”) is an indication of the magnetic materials' resistance to alteration of the magnetic orientation. A magnetic material exhibiting a vertical magnetic orientation with a high PMA strength may be less prone to alteration of its magnetic orientation out of the vertical orientation than a magnetic material exhibiting a vertical magnetic orientation with a lower magnetic strength. However, achieving a high PMA strength may not be sufficient, in and of itself, for successful STT-MRAM cell operation. For example, a low resistance-area (RA), a low switching current, a low switching voltage, and symmetric switching may also contribute to successful operation of an STT-MRAM cell. However, finding materials and designs in which a high PMA strength is exhibited without adversely affecting the other characteristics of the STT-MRAM cell's operation, particularly the RA of the cell, may present a challenge.